186 DAVIS: SECTIONS IN THE LIASSIC AND OOLITIC ROCKS OF YORKSHIRE. 
elevated above the general level on cones of sliale, which has been 
washed away all round them. The jet is probably formed of water- 
logged coniferous wood, " in some specimens the structure is preserved, in 
others entirely obliterated. A different opinion is expressed by Tate 
and Blake, f who trace the origin of jet to the bitumen occurring in 
the rocks. " It is the result of the segregation of the bitumen in the 
intervals of the shales which, allowing to a certain extent the access 
of air, has hardened it into jet, a process which may undoubtedly be 
now going on. So in cases where it is embedded in a nodule it 
remains unhardened. There seems to be no reason whatever for 
connecting it with wood, beyond its having a remotely similar com- 
position, though, of course, we have thrown no light on the cause of 
the presence of bitumen itself." It is further assumed that the 
bitumen is derived from the decayed vegetables of the period. 
The Jet Rock is, palaeontologically, especially interesting from the 
large number and variety of the fish and saurian remains found in 
it. The " scale fish" as it is locally termed, Lepidotus semiserratus, 
is the most common ; examples of other genera are also found, 
Ptijcholepis, Leptolepis, Pachycormus and Gyrosteus. Of the saurians 
examples of the Ichthyosaurus and Plesiosaurus have been found in 
the Jet workings. A number of Cephalopods, including the Aptychi, 
as well as other mollusca occur, which are not commonly found in 
other Liassic strata. The characteristic ammonite being A . serpen- 
tinus. 
Messrs. Tate and Blake give the following list of fossils from the 
Jet Rocks (zone of Am. seipcntinus) : — 
Steneosaurus brevier. Belemnites subtenuis, Simp. 
Ichthyosaurus tenuirostris. B. subaduncatus. 
Plesiosaurus longirostris. B. tripartitus, Schloth. 
P. propinquus. Ammonites heterophyllus, Lon. 
Gyrosteus mirabilis, Ag A. levisoni. 
JEchmodus ovalis, A. gracilis. 
Lepidotus semiserratus. A. crassescens. 
* Op. cit. Memoir Geol. Survey, p. 23. 
f The Yorkshire Lias, p. 178. 
